1. Field of the Invention
The present invention relates to a diversity receiver for a burst OFDM modulated signal transmission system using orthogonal frequency domain multiplex (which will be referred to as OFDM hereunder), and more particularly to a diversity receiver superior in an anti-multi-path fading characteristic.
2. Description of Related Art
The orthogonal frequency domain multiplex modulation mode is a multi-carrier modulation mode using a plurality of sub-carriers. In the OFDM modulation mode, transmission data is divided and distributed in accordance with each sub-carrier to be transmitted. Since a plurality of sub-carriers are used to transmit data, there is no need to assure a wide band for each sub-carrier. This is advantageous in the anti-multi-path fading characteristic.
The further prevalence of the data transmission system using OFDM is expected in the field of mobile radio communication. Various kinds of personal digital assistance are proposed in the field of mobile ratio communication. A user is becoming capable of performing data communication even when the user is moving at high speed, and a need for improvement in the communication performance in the mobile communication. In addition, a receiver having the excellent anti-multi-path fading characteristic in a further simple structure is desired.
Conventionally, as an example of a receiver adopting the OFDM modulation mode superior in the anti-multi-path fading characteristic, a diversity receiver such as shown in FIG. 1 has been used. The illustrated diversity receiver has been conventionally known. This is a so-called an antenna selective diversity system.
An OFDM modulated signal received by this diversity receiver is intermittently transmitted. Each intermittently transmitted signal is referred to as an OFDM burst signal and has a fixed format.
FIG. 2 shows a structure of the OFDM burst signal received by the diversity receiver depicted in FIG. 1.
Referring to FIG. 2, each OFDM burst signal is constituted by a preamble signal for antenna selection 11, a preamble signal for synchronization 12, a preamble signal for channel estimation 13, and data 14 in the mentioned order.
In FIG. 1, the OFDM modulated signals (RF signals) received by antennas 101-1 to 101-N are respectively amplified by low noise amplifiers 201-1 to 201-N and inputted to a selection circuit 1101. The selection circuit 1101 switches output signals from the low noise amplifiers 201-1 to 201-N to be sequentially selected. The output signals which are outputted from the low noise amplifiers 201-1 to 201-N and selected by the selection circuit 1101 are inputted to a frequency conversion circuit 501. The frequency conversion circuit 501 converts the output signal from the selection circuit 1101 to an IF frequency to be outputted therefrom. An OFDM demodulation circuit 601 demodulates an output signal from the frequency conversion circuit 501.
Meanwhile, the output signal from the frequency conversion circuit 501 is inputted to a reception level measurement circuit 701. The reception level measurement circuit 701 measures a level of the input signal and outputs received signal level information.
The selection circuit 1101 refers to the received signal level information outputted from the reception level measurement circuit 701 and selects an antenna whose reception level is maximum.
However, the above-described receiver adopting the OFDM modulation mode in the antenna selective diversity can not necessarily demonstrate the excellent anti-multi-path fading characteristic in the multi-path fading environment.
That is, in the multi-path fading environment, the reception level of a given sub-carrier may be lowered in some cases even if an antenna having the maximum reception level is selected. In such a case, data transmitted by using the sub-carrier having the lowered reception level can not be accurately demodulated, which may cause an error.
Accordingly, selection of an antenna having the maximum reception level can not be necessarily optimum.
It is therefore an object of the present invention to provide a diversity receiver which has a further simple structure and the excellent anti-multi-path fading characteristic.
To this end, a diversity receiver according to the present invention has a structure in which received signals of a plurality of antennas receiving burst OFDM modulated signals are selected to be synthesized.
Controlling means for selecting an antenna in accordance with the reception state of each antenna controls so as to synthesize received signals of all the antennas upon detecting the OFDM burst signal.
Further, another controlling means controls so as to turn on all ON/OFF switches of the antennas whose reception level exceeds a determined reception level to synthesize received signals upon detecting the OFDM burst signal.
Furthermore, still another controlling means measures channel delay spread of the received signal of each antenna upon detecting the OFDM burst signal and turn on all ON/OFF switches of the antennas whose delay spread is within a determined threshold value to synthesize a received signal.
Moreover, yet another controlling means measures a phase of the received signal of each antenna upon detecting the OFDM burst signal and turn on all ON/OFF switches of the antennas having the received signal with the same phase to be synthesized.
In addition, further controlling means controls so as to turn on power supplies of only low noise amplifiers of the antennas used for reception in order to reduce the consumption power.